Optical fiber design method

ABSTRACT

An object is to provide a beam propagating method capable of satisfying desired output power and a desired propagation distance and a required condition of beam quality and a method of designing an optical fiber designing the structure of an optical fiber. According to the present invention, an effective core cross-sectional area A eff  is calculated based on desired specification values and, by appropriately adjusting the structure of an optical fiber satisfying the effective core cross-sectional area and the number of modes to be propagated, the structure of the optical fiber is determined. In this way, by controlling the excitation ratio of a high-order mode at the time of coupling laser light in the optical fiber designed as above, light of high-output laser can be propagated a long distance with the beam quality maintained.

TECHNICAL FIELD

The present invention relates to a method of designing an optical fiber propagating light of high output and high quality.

BACKGROUND ART

At present, there are two types of optical fibers, namely, single-mode and multi-mode optical fibers used in the field of laser processing by using a fiber laser. In a single-mode optical fiber used in laser processing, it is generally considered that a value of M² as an index of beam quality is 2 or less. Therefore, since the beam quality of the single-mode optical fiber used to propagate emitted light of the fiber laser is better than that of the multi-mode optical fiber, the processing merit is great. However, an output power and a propagatable distance are limited by a nonlinear optical phenomenon, particularly, stimulated Raman scattering (SRS), and, for example, in the case of propagating a light wave with 1 kW or more, the propagatable distance is limited to several meters. For this reason, in some cases, the output power of the fiber laser may be propagated from several tens of meters to several hundreds of meters by using a multi-mode optical fiber and may be used for laser processing in some cases. However, in multi-mode fiber lasers, the beam quality and the value of M² as an index of the beam quality are inevitably degraded in comparison with single-mode fiber lasers.

In addition, as disclosed in Non Patent Literature 1, a single-mode fiber laser capable of obtaining an output power of about 10 kW is realized, however the length of the delivery fiber (feeding fiber) is limited to several meters. This is because, in order to suppress the SRS with the single-mode optical fiber, it is necessary to enlarge an effective area (A_(eff)) as described later, but at the same time, since the expansion is a tradeoff with the increase of a bending loss, it is necessary to expand an outer diameter of the delivery optical fiber to the order of millimeters to realize a much larger A_(eff). If the outer diameter is increased in this manner, flexibility of the optical fiber is lost, it is difficult to bend the optical fiber, and thus, there is a problem that damage or breakage easily occurs and the optical fiber is not suitable for long-distance delivery.

To summarize the above, in the related art, it is very difficult to propagate kW-class high power light with a single-mode optical fiber and a fiber laser from several tens of meters to several hundreds of meters.

In order to solve this problem, it is effective to enlarge the effective area A_(eff) (which is generally proportional to the square of a mode field diameter) which is one of parameters of the optical fiber, and for example, design of a single-mode optical fiber with various refractive index distributions disclosed in Non Patent Literatures 2 to 5 has been studied.

However, although the values of A_(eff) in the optical fiber disclosed in the cited literatures are prescribed, a propagation distance usable with respect to the output power and the like are not clarified, and it is not obvious whether the kW-class high power light can be propagated from several tens to several hundreds of meters. Furthermore, a range of the outer diameter of the optical fiber considering the flow of the fiber design and the practicality are not clarified.

CITATION LIST Non Patent Literature

-   Non Patent Literature 1: Ichige et al., “All Solid Photonic Bandgap     Fiber with Low Bending Loss and Large Effective Core Cross Section”,     IEICE Society conference, B-13-23, 2012. (in Japanese) -   Non Patent Literature 2: W. S. Wong et al., “Breaking the limit of     maximum effective area for robust single-mode propagation in optical     fibers”, Opt. Lett., vol. 30, no. 21, pp. 2855-2857, 2005. -   Non Patent Literature 3: M. Napeirala et al., “Extremely     large-mode-area photonic crystal fibre with low bending loss”, Opt.     Express, vol. 18, no. 15, pp. 15408-15418, 2010. -   Non Patent Literature 4: M. Kashiwagi et al., “Effectively     single-mode all-solid photonic bandgap fiber with large effective     area and low bending loss for compact high-power all-fiber lasers”,     Opt. Express, vol. 20, no. 14, pp. 15061-15070, 2012. -   Non Patent Literature 5: M. C. Swan et al., “33 μm Core effectively     single-mode chirally-coupled-core fiber laser at 1064-nm”, OFC2008,     OWU2, 2008. -   Non Patent Literature 6: H. Yoda et al., “Beam quality factor of     higher order modes in a step-index fiber”, J. Lightwave Technol.,     vol. 24, no. 3, pp. 1350-1355, 2006. -   Non Patent Literature 7: G. P. Agrawal, “Nonlinear Fiber Optics”,     ACADEMIC PRESS. -   Non Patent Literature 8: F. A. Oguama et al., “Simultaneous     measurement of the Raman gain coefficient and the nonlinear     refractive index of optical fibers: theory and experiment”, J. Opt.     Soc. Am. B, vol. 22, no. 2, pp. 426-436, 2005. -   Non Patent Literature 9: Kunimasa Saitoh, Masanori Koshiba,     “Full-vectorial finite element beam propagation method with     perfectly matched layers for anisotropic optical waveguides”,     IEEE J. Lightwave technol., vol. 19, no. 3, pp. 405-413, 2001. -   Non Patent Literature 10: Kunimasa Saitoh, Masanori Koshiba,     “Emprical relations for simple design of photonic crystal fibers”,     Optical Society of America, Optics Express, 10 Jan. 2005, Vol. 13,     No. 1, pp. 267-274.

SUMMARY OF INVENTION Technical Problem

As described above, the structure of the optical fiber that can satisfy the requirements for an output power, a propagation distance, and a beam quality required in the examination examples in the related art has not been clarified.

Therefore, an object of the present invention is to provide an optical fiber design method of designing an optical fiber that can satisfy desired requirements of an output power, a propagation distance, and a beam quality and the optical fiber.

Solution to Problem

In order to achieve the above-described object, A_(eff) is calculated from desired specifications of an optical fiber, a fiber structure is provisionally determined, and it is decided that the fiber structure is corrected in consideration of a relationship with a bending loss value in a fundamental mode or higher-order mode.

More specifically, the optical fiber design method according to the present invention includes:

a specification value determining step of determining fiber loss and Raman gain coefficient of a photonic crystal fiber (PCF) to be used, a wavelength of propagating light, a beam quality M² after PCF propagation, a laser output power value, a propagation distance, and a minimum bending radius;

maximum number of propagation modes calculating step of calculating the number n of propagation modes that can be propagated by using Mathematical Formula 1;

an effective area calculating step of calculating an effective area A_(eff) from the fiber loss and the Raman gain coefficient by using Mathematical Formula 2;

a fiber structure calculating step of calculating diameter d and interval Λ of air holes of the PCF satisfying the A_(eff);

a bending loss calculating step of calculating a bending loss at the minimum bending radius in a PCF having a structure calculated in the fiber structure calculating step and calculating a bending loss at a propagation length from the propagation distance;

a checking step of checking that the bending loss at the propagation length is less than a predetermined value and determining the structure of the PCF calculated in the fiber structure calculating step; and

a mode increasing step of, in a case where the bending loss at the propagation length is equal to or more than the predetermined value in the checking step, repeating the fiber structure calculating step, the bending loss calculating step, and the checking step by increasing the number of modes by one until the number of modes reaches the number n of propagation modes.

Mathematical Formulas 1 and 2 will be described later.

In an optical fiber design method according to the present invention, a necessary A_(eff) of the optical fiber is calculated from requirements, a structure of a photonic crystal fiber (PCF) satisfying this is provisionally set, and the structure of the PCF is finely adjusted so that a bending loss becomes less than a specified value.

In addition, the fiber structure may be determined by using the following method.

Namely, in an optical fiber design method according to the present invention includes:

a specification value determining step of determining a fiber loss and Raman gain coefficient of a photonic crystal fiber (PCF) to be used, a wavelength of propagating light, a beam quality M2 after PCF propagation, a laser output power value, a propagation distance, and a minimum bending radius;

maximum number of propagation modes calculating step of calculating the number n of propagation modes that can be propagated by using Mathematical Formula 1;

an effective area calculating step of calculating an effective area A_(eff) from the fiber loss and the Raman gain coefficient by using Mathematical Formula 2;

a fiber structure calculating step of calculating diameter d and interval Λ of air holes of the PCF having the A_(eff) or more and plotting points having the A_(eff) or more on a graph of which the horizontal axis is d/Λ and of which the vertical axis is Λ;

a bending loss calculating step of calculating a bending loss at a minimum bending radius of a smallest higher-order mode cut off by the PCF from the diameter d and the interval Λ of the air holes of the PCF and plotting points having the bending loss of 1 dB/m or more on a graph of which the horizontal axis is d/Λ and of which the vertical axis is Λ; and

a structure determining step of detecting an overlapping range where a region of the points plotted on the graph in the fiber structure calculating step and a region of the points plotted on the graph in the bending loss calculating step overlap each other and determining a PCF structure having air holes having diameters d and intervals Λ in the overlapping range.

Mathematical Formulas 1 and 2 will be described later.

In the design of the PCF, the PCF has air holes having diameters d and intervals Λ in an overlapping region where a region of A_(eff) of a desired value or more and a cutoff region in a desired higher order overlap each other on a graph where the horizontal axis represents d/Λ and the vertical axis represents Λ, so that it is possible to sufficiently cut off the mode which is the desired higher-order mode or more, and thus, it is possible to select a region where the A_(eff) is large.

Therefore, according to the present invention, it is possible to provide an optical fiber design method capable of satisfying desired requirements of an output power, a propagation distance, and a beam quality.

This is an example of a PCF designed by the optical fiber design method.

The PCF is a PCF having a one-cell structure in which air holes of a diameter d are arranged at an interval Λ in a cross-section and has a feature in which, when the coordinates are represented as coordinates (d/Λ, Λ), air holes of a diameter d and an interval Λ that are in an area surrounded by a polygon having

A1(0.42, 16.88),

B(0.42, 10.94),

C1(0.60, 15.63),

C2(0.69, 31.88),

D(0.74, 43.12),

E(0.75, 44.38),

C3(0.76, 47.81),

F(0.81, 60.63),

G(0.85, 60.63),

H(0.85, 77.50), and

I(0.90, 91.88)

as its vertexes are included.

Thus, A_(eff) can be configured to be as large as possible while the first high-order mode and higher modes are sufficiently cut off.

The following is an example of a PCF that is designed using the above-described method of designing an optical fiber.

The PCF is a PCF having a one-cell structure in which air holes of a diameter d are arranged at an interval Λ in a cross-section and has a feature in which, when the coordinates are represented as coordinates (d/Λ, Λ), air holes of a diameter d and an interval Λ that are in an area surrounded by a polygon having

A1(0.42, 16.88),

B(0.42, 10.94),

C1(0.75, 15.00),

D1(0.75, 20.00),

C2(0.78, 35.00),

D2(0.80, 35.93),

E(0.80, 45.63),

F(0.83, 51.56),

C3(0.90, 54.38), and

I(0.90, 91.88)

as its vertexes are included.

Thus, A_(eff) can be configured to be as large as possible while the third high-order mode and higher modes are sufficiently cut off.

The following is another example of a PCF that is designed using the above-described method of designing an optical fiber.

The PCF is a PCF having a one-cell structure in which air holes of a diameter d are arranged at an interval Λ in a cross-section and has a feature in which, when the coordinates are represented as coordinates (d/Λ, Λ), air holes of a diameter d and an interval Λ that are in an area surrounded by a polygon having

A1(0.42, 16.88),

B(0.42, 10.94),

C(0.75, 14.24),

D(0.75, 12.10),

E(0.79, 20.00),

F(0.85, 30.00),

G(0.85, 41.58),

H(0.89, 50.00),

I(0.89, 58.95),

J(0.90, 60.0), and

K(0.90, 91.88)

as its vertexes are included.

Thus, A_(eff) can be configured to be as large as possible while the fourth high-order mode and higher modes are sufficiently cut off.

The following is another example of a PCF that is designed using the above-described method of designing an optical fiber.

The PCF is a PCF having a seven-cell structure in which air holes of a diameter d are arranged at an interval Λ in a cross-section and has a feature in which, when the coordinates are represented as coordinates (d/Λ, Λ), air holes of a diameter d and an interval Λ that are in an area surrounded by a polygon having

A1(0.20, 10.98),

B(0.20, 4.95),

C1(0.25, 5.27),

D(0.29, 9.87),

E(0.40, 12.25),

F(0.40, 13.52),

G(0.49, 14.15),

H(0.49, 15.74),

I(0.50, 18.12),

J(0.58, 18.12),

K(0.58, 19.86),

C3(0.60, 20.34),

L(0.60, 23.03),

M(0.68, 23.99),

N(0.68, 31.60),

O(0.79, 48.73), and

P(0.80, 50.00)

as its vertexes are included.

Thus, A_(eff) can be configured to be as large as possible while the first high-order mode and higher modes are sufficiently cut off.

The following is another example of a PCF that is designed using the above-described method of designing an optical fiber.

The PCF is a PCF having a seven-cell structure in which air holes of a diameter d are arranged at an interval Λ in a cross-section and has a feature in which, when the coordinates are represented as coordinates (d/Λ, Λ), air holes of a diameter d and an interval Λ that are in an area surrounded by a polygon having

A1(0.20, 10.98),

B(0.20, 5.11),

C1(0.40, 5.90),

D(0.40, 10.03),

E(0.50, 11.93),

F(0.50, 14.47),

C3(0.60, 20.18),

G(0.69, 22.08),

H(0.68, 23.67,),

I(0.70, 24.30),

J(0.70, 32.87), and

K(0.80, 50.00)

as its vertexes are included.

Thus, A_(eff) can be configured to be as large as possible while the third high-order mode and higher modes are sufficiently cut off.

The following is another example of a PCF that is designed using the above-described method of designing an optical fiber.

The PCF is a PCF having a seven-cell structure in which air holes of a diameter d are arranged at an interval Λ in a cross-section and has a feature in which, when the coordinates are represented as coordinates (d/Λ, Λ), air holes of a diameter d and an interval Λ that are in an area surrounded by a polygon having

A1(0.20, 10.98),

B(0.20, 5.11),

C(0.50, 6.23),

D(0.50, 10.00),

E(0.60, 15.18),

F(0.60, 17.76),

G(0.65, 20.12),

H(0.70, 20.35),

I(0.79, 25.06),

J(0.79, 29.53),

K(0.78, 29.76),

L(0.78, 38.29),

M(0.80, 40.12), and

N(0.80, 50.00)

as its vertexes are included.

Thus, A_(eff) can be configured to be as large as possible while the fourth high-order mode and higher modes are sufficiently cut off.

A light transmission using the above-described PCF will be described.

It is the light transmission that propagates light with 1 kW or more from a laser for 10 m or more and outputting the light from the output end,

the laser and the output end are connected with a 2-mode fiber of which the number of propagation modes is 2 at the wavelength of the light, and

the light is propagated by setting an excitation ratio of the first higher-order mode of the 2-mode fiber to be 50% or less.

The 2-mode fiber has larger A_(eff) compared with the single-mode fiber. For this reason, since the SRS can be suppressed, the propagation distance of the high-power light can be extended. Furthermore, since the excitation ratio of the first higher-order mode of the 2-mode fiber can be adjusted by the optical axis between the laser and the optical fiber, it is possible to transmit the high-power light with desired beam quality.

More specifically, the light transmission according to the present invention is a light transmission of propagating light with 1 kW or more from a laser for 10 m or more and outputting the light from the output end,

the laser and the output end are connected with a 4-mode fiber of which the number of propagation modes is 4 or less at wavelength of the light, and

the light is propagated by setting an excitation ratio of the third higher-order mode of the 4-mode fiber to be 30% or less.

The 4-mode fiber has larger A_(eff) compared with the single-mode fiber. For this reason, since the SRS can be suppressed, the propagation distance of high-power light can be extended. Furthermore, since the excitation ratio of the first higher-order mode and the second higher-order mode of the 4-mode fiber can be reduced by the optical axis between the laser and the optical fiber and the excitation ratio of the third higher-order mode can be adjusted by a mode field diameter of the optical fiber and a spot size of the light coupled to the optical fiber, it is possible to transmit the high power light with desired beam quality.

Therefore, it is possible to realize a light transmission capable of satisfying desired requirements of an output power, a propagation distance, and a beam quality in a PCF designed by the optical fiber design method according to the present invention.

Advantageous Effects of Invention

According to the present invention, it is possible to provide an optical fiber design method of designing an optical fiber that can satisfy desired requirements of an output power, a propagation distance, and a beam quality and the optical fiber.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram that illustrates examples of waveguide modes of an optical fiber.

FIG. 2 is a diagram that illustrates excitation efficiency with respect to an axial deviation from LP01 to LP11.

FIG. 3 is a diagram that illustrates an example of an MFD difference of an optical fiber and efficiency of excitation from a fundamental mode to a third high-order mode.

FIG. 4 is a diagram that illustrates an example of the cross-section of a PCF having a one-cell structure.

FIG. 5 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) satisfying a bending loss of 0.1 dB/m or less when light having a wavelength of 1070 nm is propagated in the fundamental mode in a PCF having the one-cell structure.

FIG. 6 is a diagram that illustrates an area (cut-off area) in which a bending loss of a first high-order (LP11) mode of light having a wavelength of 1070 nm in a PCF having the one-cell structure is 1 dB/m or more.

FIG. 7 is a diagram that illustrates an area (cut-off area) in which a bending loss of a second high-order (LP21) mode of light having a wavelength of 1070 nm in a PCF having the one-cell structure is 1 dB/m or more.

FIG. 8 is a diagram that illustrates an area (cut-off area) in which a bending loss of a third high-order (LP02) mode of light having a wavelength of 1070 nm in a PCF having the one-cell structure is 1 dB/m or more.

FIG. 9 is a diagram that illustrates an area (cut-off area) in which a bending loss of a fourth high-order (LP31) mode of light having a wavelength of 1070 nm in a PCF having the one-cell structure is 1 dB/m or more.

FIG. 10 is a diagram that illustrates the structure dependency of the fundamental mode bending loss of a PCF having the one-cell structure for 1050 nm and 1070 nm.

FIG. 11 is a diagram that illustrates an example of the cross-section of a PCF having a seven-cell structure.

FIG. 12 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) satisfying a bending loss of 0.1 dB/m or less when light having a wavelength of 1070 nm is propagated in the fundamental mode in a PCF having the seven-cell structure.

FIG. 13 is a diagram that illustrates an area (cut-off area) in which a bending loss of the first high-order (LP11) mode of light having a wavelength of 1070 nm in a PCF having the seven-cell structure is 1 dB/m or more.

FIG. 14 is a diagram that illustrates an area (cut-off area) in which a bending loss of the second high-order (LP21) mode of light having a wavelength of 1070 nm in a PCF having the seven-cell structure is 1 dB/m or more.

FIG. 15 is a diagram that illustrates an area (cut-off area) in which a bending loss of the third high-order (LP02) mode of light having a wavelength of 1070 nm in a PCF having the seven-cell structure is 1 dB/m or more.

FIG. 16 is a diagram that illustrates an area (cut-off area) in which a bending loss of the fourth high-order (LP31) mode of light having a wavelength of 1070 nm in a PCF having the seven-cell structure is 1 dB/m or more.

FIG. 17 is a diagram that illustrates the structure dependency of the bending loss of the fundamental mode of a PCF having the seven-cell structure for 1050 nm and 1070 nm.

FIG. 18 is a flowchart that illustrates a method of designing an optical fiber according to the present invention.

FIG. 19 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) (160 μm² or more) satisfying (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is four or less when light having a wavelength of 1070 nm is propagated in a PCF having the one-cell structure. An area denoted by diagonal lines is an area in which 100 W output and 300 m propagation can be performed.

FIG. 20 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) (800 μm² or more) satisfying (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is four or less when light having a wavelength of 1070 nm is propagated in a PCF having the one-cell structure. An area denoted by diagonal lines is an area in which 500 W output and 300 m propagation can be performed.

FIG. 21 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) (1600 μm² or more) satisfying (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is four or less when light having a wavelength of 1070 nm is propagated in a PCF having the one-cell structure. An area denoted by diagonal lines is an area in which 1000 W output and 300 m propagation can be performed.

FIG. 22 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) (3200 μm² or more) satisfying (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is four or less when light having a wavelength of 1070 nm is propagated in a PCF having the one-cell structure. An area denoted by diagonal lines is an area in which 600 kW·m propagation can be performed. An area satisfying conditions (a) and (b) is not present.

FIG. 23 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) (160 μm² or more) satisfying (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is four or less when light having a wavelength of 1070 nm is propagated in a PCF having the seven-cell structure. An area denoted by diagonal lines is an area in which 100 W output and 300 m propagation can be performed.

FIG. 24 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) (800 μm² or more) satisfying (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is four or less when light having a wavelength of 1070 nm is propagated in a PCF having the seven-cell structure. An area denoted by diagonal lines is an area in which 500 W output and 300 m propagation can be performed.

FIG. 25 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) (1600 μm² or more) satisfying (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is four or less when light having a wavelength of 1070 nm is propagated in a PCF having the seven-cell structure. An area denoted by diagonal lines is an area in which 1000 W output and 300 m propagation can be performed.

FIG. 26 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) (3200 μm² or more) satisfying (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is four or less when light having a wavelength of 1070 nm is propagated in a PCF having the seven-cell structure. An area denoted by diagonal lines is an area in which 2000 W output and 300 m propagation can be performed.

FIG. 27 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) (4800 μm² or more) satisfying (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is four or less when light having a wavelength of 1070 nm is propagated in a PCF having the seven-cell structure. An area denoted by diagonal lines is an area in which 3000 W output and 300 m propagation can be performed.

FIG. 28 is a diagram that illustrates another example of the cross-section of a PCF having the one-cell structure.

FIG. 29 is a diagram that illustrates a relation among d/Λ, Λ, and A_(eff) satisfying a bending loss of 0.1 dB/m or less when light having a wavelength of 1070 nm is propagated in a PCF having the one-cell structure.

FIG. 30 is a diagram that illustrates an area (cut-off area) in which a bending loss of the first high-order (LP11) mode of light having a wavelength of 1070 nm in a PCF having the one-cell structure is 1 dB/m or more.

FIG. 31 is a diagram that illustrates an area (cut-off area) in which a bending loss of the second high-order (LP21) mode of light having a wavelength of 1070 nm in a PCF having the one-cell structure is 1 dB/m or more.

FIG. 32 is a diagram that illustrates an area (cut-off area) in which a bending loss of the third high-order (LP02) mode of light having a wavelength of 1070 nm in a PCF having the one-cell structure is 1 dB/m or more.

FIG. 33 is a diagram that illustrates a relation between d/Λ and A satisfying M²≤2.0 when light having a wavelength of 1070 nm is propagated in a PCF having the one-cell structure.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described with reference to the attached drawings. The embodiments described below are examples of the present invention, and thus, the present invention is not limited to the embodiments presented below. In the following description and the drawings, like reference numerals denote like elements.

Embodiment 1

Characteristics of the present invention are to increase an output power and to extend a propagation distance by enlarging A_(eff). A major difference of the present invention from the related art is that an optical fiber has a structure capable of propagating a plurality of modes. The present invention sufficiently reduces an efficiency at which the higher-order mode is excited in an input unit and clarifies a region where M² can be made sufficiently small, so that A_(eff) can be set to a value that cannot be realized in the related art.

First, the value of M² that is an index of beam quality is described below. An embodiment of the present invention will be described based on each waveguide mode illustrated in FIG. 1.

The M² values for the waveguide modes of the optical fiber are disclosed to be 1.1 for the fundamental mode, 3.3 for the first higher-order mode, 3.3 for the second higher-order mode, and 3.1 for the third higher-order mode in Non Patent Literature 6. In addition, it is disclosed that, in a case where the fundamental mode and the first higher-order mode coexist, the M² value varies depending on a phase relationship between an excitation ratio of the first higher-order mode and the fundamental mode, and it can be understood that, if the excitation ratio of the first higher-order mode is lower than about 50%, the M² value becomes 2.0 or less.

A first beam propagation method according to the embodiment is a light transmission of propagating light with 1 kW or more from a laser for 10 m or more and outputting the light from an output end,

the laser and the output end are connected with a 2-mode fiber of which the number of propagation modes is 2 at the wavelength of the light, and

the light is propagated by setting an excitation ratio of the first higher-order mode of the 2-mode fiber to be 50% or less.

In the first higher-order mode, the excitation ratio varies depending on an amount of axis shift from the center of the optical fiber when light is generally input to the optical fiber. In addition, reduction of the excitation ratio of the first higher-order mode down to 50% or less can be sufficiently realized with existing optical input/output alignment (optical axis alignment) technology.

Accordingly, if a 2-mode fiber in which the second higher-order mode is cut off and only the fundamental mode and the first higher-order mode exist is used, by appropriately controlling the excitation ratio, high-quality beam propagation with an M² value of 2.0 or less which cannot be realized with the multi-mode fiber in the related art becomes possible, and fiber design greatly exceeding A_(eff) which cannot be realized with the single-mode fiber in the related art becomes possible. In addition, the worst value of the M² value of the 2-mode fiber in which only the fundamental mode and the first higher-order mode exist is 3.3, and fiber design of enabling relatively high-quality beam propagation in comparison with the multi-mode optical fiber is possible.

The basis of the worst value of the M² value of the 2-mode fiber is illustrated in FIG. 4 of Non Patent Literature 6. In this figure, M² denotes the excitation ratio of the LP11 mode of a case where the LP01 (fundamental mode) and the LP11 mode (first higher-order mode) coexist, and M² is 3.3 when a (excitation ratio of the LP11 mode) is 1.0. In addition, unless the LP11 mode is intentionally excited, the LP11 mode will not be excited 100%, so that the worst value of the M² value of the 2-mode fiber is 3.3.

A second beam propagation method according to the embodiment is a light transmission of propagating light with 1 kW or more from a laser for 10 m or more and outputting the light from the output end,

the laser and the output end are connected with a 4-mode fiber of which the number of propagation modes is 4 at the wavelength of light, and

the light is propagated by setting an excitation ratio of the third higher-order mode of the 4-mode fiber to be 30% or less.

Non Patent Literature 6 also discloses an M² value when the fundamental mode and the third higher-order mode coexist, and thus, it can be understood that, if the excitation ratio of the third higher-order mode is lower than about 30%, the M² value becomes 2 or less. The fundamental mode and the third higher-order mode are modes having the electric field peak at the center of the fiber, and the coupling efficiency varies depending on a spot size (MFD 2) of the light input to the fiber having a mode field diameter (MFD 1). FIG. 3 is a diagram illustrating a relationship between an MFD difference and an efficiency (rate of occurrence of the third higher-order mode) of excitation of the third higher-order mode when the vertical axis represents the efficiency of excitation of the third higher-order mode and the horizontal axis represents the MFD difference ((MFD2−MFD1)/MFD1) input to the fiber.

From FIG. 3, it is possible to reduce the occurrence of the third higher-order mode to about 30% or less by suppressing the MFD difference within 20% or by inputting a beam having a spot size more than the MFD 1 of the fiber, so that the M² value can be reduced to 2 or less. More specifically, since the third higher-order mode occurs according to the distance between the optical fibers (corresponding to a difference in beam diameter between input light and a fundamental mode of a 4-mode optical fiber), the distance between the fiber laser and the 4-mode optical fiber is adjusted. In addition, the first higher-order mode and the second higher-order mode are higher-order modes which occur due to axis shift of the fiber laser and the optical fiber and can be sufficiently suppressed from results of an axis shift and an excitation amount of the first higher-order mode.

From the above description, by appropriately controlling the excitation ratio by using the 4-mode fiber where the fourth higher-order mode is cut off, it is possible to design a fiber where high-quality beam propagation with an M² value of 2.0 or less is possible. Furthermore, since the worst value of the M² value of the 4-mode fiber is 3.3, it is possible to design a fiber that enables relatively high-quality beam propagation with an M² value of at worst 3.3 or less.

In addition, the ground for the worst value of the M² value of the 2-mode fiber is illustrated in FIG. 6 of Non Patent Literature 6. In this figure, M² is illustrated with respect to the excitation ratio of LP02 mode in a case where LP01 (fundamental mode) and LP02 mode (third higher-order mode) coexist, and when a (excitation ratio of LP02 mode) is 0.9, the M^(Z) is 3.3. Therefore, the worst value of the M² value of the 4-mode fiber is 3.3.

In the propagation of only the single-mode, there exists a limit to enlargement of A_(eff), but as in the light transmission described in the embodiment, the propagation of several modes is permitted and the design range is widened by using a 2-mode fiber or a 4-mode fiber, so that it is possible to realize the A_(eff) which does not exist in the related art. Therefore, by using the light transmission according to the embodiment, it is possible to propagate light satisfying the desired requirements of an output power, a propagation distance, and a beam quality.

Embodiment 2

This embodiment relates to a design method of structural parameters (diameters d of the air hole 2 and intervals Λ between the air holes 2) for realizing enlargement of an effective area A_(eff) and implementing a predetermined bending loss αb in a PCF having a 1-cell core structure having air holes 2 illustrated in FIG. 4. In the embodiment, a minimum bending radius in a fundamental mode and a bending radius defining an effective cutoff of a higher-order mode are described as 140 mm, but the method according to the embodiment is not limited thereto.

The design method includes:

a specification value determining step of determining fiber loss and Raman gain coefficient of a photonic crystal fiber (PCF) to be used, a wavelength of propagating light, a beam quality M² after PCF propagation, a laser output power value, a propagation distance and a minimum bending radius;

propagation modes maximum number of propagation modes calculating step of calculating the number n of propagation modes that can be propagated by using Mathematical Formula 1;

an effective area calculating step of calculating an effective area A_(eff) from the fiber loss and the Raman gain coefficient by using Mathematical Formula 2;

a fiber structure calculating step of calculating diameter d and interval Λ of air holes of the PCF having the A_(eff) or more and plotting points having the A_(eff) or more on a graph of which the horizontal axis is d/Λ and of which the vertical axis is Λ;

a bending loss calculating step of calculating a bending loss at a minimum bending radius of a smallest higher-order mode cut off by the PCF from the diameter d and the interval Λ of the air holes of the PCF and plotting points having the bending loss of 1 dB/m or more on a graph of which the horizontal axis is d/Λ and of which the vertical axis is Λ; and

a structure determining step of detecting an overlapping range where a region of the points plotted on the graph in the fiber structure calculating step and a region of the points plotted on the graph in the bending loss calculating step overlap each other and determining a PCF structure having air holes having diameters d and intervals Λ in the overlapping range.

Mathematical Formulas 1 and 2 will be described later.

As illustrated in FIG. 4, the optical fiber according to the embodiment is a photonic crystal fiber (PCF) having a 1-cell structure which has a core portion and a cladding portion that surrounds the core portion, wherein the core portion and the cladding portion are made of a medium having a uniform optical refractive index, and a plurality of uniform air holes 2 are formed in the cladding portion in the longitudinal direction. In addition, the 1-cell structure in the present invention denotes a structure of a photonic crystal having one defect, in which only the air holes in the central portion of the photonic crystal formed with the air holes arranged in a triangular lattice shape are made of quartz.

FIG. 5 is a diagram illustrating a range of the A_(eff) of the PCF in the region where the bending loss in the fundamental mode with a bending radius of 140 mm is 1.0 dB/m or less with respect to light having a wavelength of 1070 nm when the horizontal axis represents d/Λ and the vertical axis represents Λ. The bending radius and the bending loss value in the fundamental mode are not limited to the definition and the specified value according to the present invention, but it is possible to determine the parameters used for the design according to the required characteristics. The A_(eff) is obtained by using the following Mathematical Formula.

$\begin{matrix} {\;\left\lbrack {{Numerical}\mspace{14mu}{Expression}\mspace{14mu} 3} \right\rbrack} & \; \\ {A_{eff} = \frac{\left\lbrack {\int{\int{{{E\left( {x,y} \right)}}^{2}{dx}\;{dy}}}} \right\rbrack^{2}}{\int{\int{{{E\left( {x,y} \right)}}^{4}{dx}\;{dy}}}}} & (3) \end{matrix}$

Herein, E is the electric field of light, and x and y are the coordinates in the fiber cross section (assumed to be the xy plane).

FIG. 6 illustrates the bending loss in the first higher-order mode at a bending radius of 140 mm of the 1-cell structure PCF with respect to light having a wavelength of 1070 nm when the horizontal axis represents d/Λ and the vertical axis represents Λ. The plotted range in the figure illustrates the bending loss in the first higher-order mode in the range of 1 dB/m or more (the range in which the cutoff is obtained). By setting to d/Λ and Λ where the range illustrated in FIG. 5 and the range illustrated in FIG. 6 overlap each other, the first higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where the A_(eff) is large. For example, when the coordinates are represented by (d/Λ, Λ), the air holes are set to d and Λ in a region surrounded by a polygon having four vertices

A(0.42, 16),

B(0.42, 10),

C(0.53, 10), and

D(0.80, 56).

More specifically, it can be understood from FIG. 6 that, if d/Λ is 0.724 and Λ is about 45 μm, A_(eff) is 1400 μm², and the bending loss in the first higher-order mode is 40 dB/m or more, namely, the structure which d/Λ is 0.724 and Λ is about 45 μm becomes a single mode effectively.

FIG. 7 illustrates the bending loss (1 dB/m or more) in the second higher-order mode at a bending radius of 140 mm of the 1-cell structure PCF with respect to light having a wavelength of 1070 nm when the horizontal axis represents d/Λ and the vertical axis represents Λ. By setting to d/Λ and Λ where the range illustrated in FIG. 5 and the range illustrated in FIG. 7 overlap each other, the second higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where the A_(eff) is large. For example, when the coordinates are represented by (d/Λ, Λ), the air holes are set to d and Λ in a region surrounded by a polygon having four vertices

A(0.42, 16),

B(0.42, 10),

C(0.76, 10), and

D(0.80, 56).

In addition, FIG. 8 illustrates the bending loss (1 dB/m or more) in the third higher-order mode at a bending radius of 140 mm of the 1-cell structure PCF with respect to light having a wavelength of 1070 nm when the horizontal axis represents d/Λ and the vertical axis represents Λ. By setting to d/Λ and Λ where the range illustrated in FIG. 5 and the range illustrated in FIG. 8 overlap each other, the third higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where the A_(eff) is large. For example, when the coordinates are represented by (d/Λ, Λ), the air holes are set to d and Λ in a region surrounded by a polygon having four vertices

A(0.42, 16),

B(0.42, 10),

C(0.76, 10), and

D(0.80, 56).

In addition, FIG. 9 illustrates the bending loss (1 dB/m or more) in the fourth higher-order mode at a bending radius of 140 mm of the 1-cell structure PCF with respect to light having a wavelength of 1070 nm when the horizontal axis represents d/Λ and the vertical axis represents Λ. By setting to d/Λ and Λ where the range illustrated in FIG. 5 and the range illustrated in FIG. 9 overlap each other, the fourth higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where the A_(eff) is large. For example, when the coordinates are represented by (d/Λ, Λ), the air holes are set to d and Λ in a region surrounded by a polygon having four vertices

A(0.42, 16),

B(0.42, 10),

C(0.76, 10), and

D(0.80, 56).

By setting to d/Λ and Λ where the range illustrated in FIG. 5 and the range illustrated in FIG. 7, FIG. 8 or FIG. 9 overlap each other as described above, the second higher-order mode or more, the third higher-order mode or more, or the fourth higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where the A_(eff) is large.

FIG. 10 illustrates wavelength dependency of a bending loss of a PCF with a 1-cell structure. In the figure, the solid line (open circles) and the broken line (black circles) illustrate air hole interval (Λ) dependency of the bending loss in the fundamental mode for the wavelength 1050 nm and the wavelength 1070 nm, respectively, as, and the values in parentheses in the figure indicate values of d/Λ for each Λ. It is generally known that the PCF has characteristics in that the A_(eff) has almost the same value regardless of the propagation wavelength and the bending loss increases as the wavelength becomes shorter. FIG. 10 illustrates that the structure illustrated in this design is realized in any structure where the bending loss in the fundamental mode is 0.1 dB/m or less even at a wavelength of 1050 nm and that the present design is effective at a wavelength of 1050 nm or more.

Embodiment 3

This embodiment relates to a design method of structural parameters (diameters d of the air hole 2 and intervals Λ between the air holes 2) for realizing enlargement of A_(eff) and implementing a predetermined bending loss αb in a PCF having a 7-cell core structure having a plurality of air holes 2 as illustrated in FIG. 11. This design method also employs the design method described in the second embodiment.

As illustrated in FIG. 11, the optical fiber according to the embodiment is a PCF having a 7-cell structure which has a core portion and a cladding portion that surrounds the core portion, wherein the core portion and the cladding portion are made of a medium having a uniform optical refractive index, and a plurality of uniform air holes 2 are formed in the cladding portion in the longitudinal direction. In addition, the 7-cell structure in the present invention denotes a structure of a photonic crystal having seven defects, in which one air hole in the central portion of the photonic crystal formed with the air holes arranged in a triangular lattice shape and six air holes around the air hole are made of quartz.

FIG. 12 is a diagram illustrating A_(eff) in a region where the bending loss in the fundamental mode at a bending radius of 140 mm is 1.0 dB/m or less when the horizontal axis represents d/Λ and the vertical axis represents Λ. Since the PCF according to the embodiment has a 7-cell structure, in comparison with the 3-layer structure of the 1-cell structure of Embodiment 1, it is advantageous in that the fiber outer diameter can be reduced in a region where A_(eff) is 1000 μm² or more. In the 3-layer 1-cell structure, a cladding diameter of 500 μm is required to achieve the A_(eff) of 1000 μm², whereas in the 4-layer 7-cell structure, it can be realized with an outer diameter of about 200 μm. The definition of the bending loss value and cutoff in the fundamental mode is not limited to the definition and the specified value of the present invention, but it is possible to determine parameters used for the design according to the required characteristics.

FIG. 13 illustrates the bending loss in the first higher-order (LP11) mode at a bending radius of 140 mm of the 7-cell structure PCF with respect to light having a wavelength of 1070 nm when the horizontal axis represents d/Λ and the vertical axis represents Λ. The plotted range in the figure illustrates the bending loss in the first higher-order mode in the range of 1 dB/m or more (the range in which the cutoff is obtained). By setting to d/Λ and Λ where the range illustrated in FIG. 12 and the range illustrated in FIG. 13 overlap each other, the first higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where the A_(eff) is large. For example, when the coordinates are represented by (d/Λ, Λ), the air holes are set to d/Λ and Λ in a region surrounded by a triangle having three vertices

A(0.20, 7.80),

B(0.34, 10.82), and

C(0.78, 48.42).

More specifically, it can be understood that, if d/Λ is 0.68 and Λ is about 40 μm, A_(eff) is 5700 μm² and the bending loss in the first higher-order mode is 20 dB/m or more, namely, the structure which d/Λ is 0.68 and Λ is about 40 μm becomes a single mode effectively.

FIG. 14 illustrates the bending loss (1 dB/m or more) in the second higher-order mode at a bending radius of 140 mm of the 7-cell structure PCF with respect to light having a wavelength of 1070 nm when the horizontal axis represents d/Λ and the vertical axis represents Λ. By setting to d/Λ and Λ where the range illustrated in FIG. 12 and the range illustrated in FIG. 14 overlap each other, the second higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where the A_(eff) is large. For example, when the coordinates are represented by (d/Λ, Λ), the air holes are set to d/Λ and Λ in a region surrounded by a polygon having four vertices

A(0.20, 7.80),

B(0.20, 4.00),

C(0.80, 4.00), and

D(0.80, 50.0).

In addition, FIG. 15 illustrates the bending loss (1 dB/m or more) in the third higher-order mode at a bending radius of 140 mm of the 1-cell structure PCF with respect to light having a wavelength of 1070 nm when the horizontal axis represents d/Λ and the vertical axis represents Λ. By setting to d/Λ and Λ where the range illustrated in FIG. 12 and the range illustrated in FIG. 15 overlap each other, the third higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where the A_(eff) is large. For example, when the coordinates are represented by (d/Λ, Λ), the air holes are set to d/Λ and Λ in a region surrounded by a polygon having four vertices

A(0.20, 7.80),

B(0.20, 4.00),

C(0.80, 4.00), and

D(0.80, 50.0).

In addition, FIG. 16 illustrates the bending loss (1 dB/m or more) in the fourth higher-order mode at a bending radius of 140 mm of the 1-cell structure PCF with respect to light having a wavelength of 1070 nm when the horizontal axis represents d/Λ and the vertical axis represents Λ. By setting to d/Λ and Λ where the range illustrated in FIG. 12 and the range illustrated in FIG. 16 overlap each other, the fourth higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where A_(eff) is large. For example, when the coordinates are represented by (d/Λ, Λ), the air holes are set to d/Λ and Λ in a region surrounded by a polygon having four vertices

A(0.20, 7.80),

B(0.20, 4.00),

C(0.80, 4.00), and

D(0.80, 50.0).

As described above, by setting to d/Λ and Λ where the range illustrated in FIG. 12 and the range illustrated in FIG. 14, FIG. 15 or FIG. 16 overlap each other, the second higher-order mode or more, the third higher-order mode or more, or the fourth higher-order mode or more is sufficiently cut off, and thus, it is possible to select a region where the A_(eff) is large.

FIG. 17 illustrates wavelength dependency of a bending loss of a PCF with a 7-cell structure. In the figure, the solid line (open circles) and the broken line (black circles) illustrate air hole interval Λ dependency of the bending loss in the fundamental mode for the wavelength 1050 nm and the wavelength 1070 nm, respectively, as, and the values in parentheses in the figure indicate values of d/Λ for each Λ. It is generally known that the PCF has characteristics in that the A_(eff) has almost the same value regardless of the propagation wavelength and the bending loss increases as the wavelength becomes shorter. FIG. 17 illustrates that the structure illustrated in this design is realized in any structure where the bending loss in the fundamental mode is 0.1 dB/m or less even at a wavelength of 1050 nm and that the present design is effective at a wavelength of 1050 nm or more.

Embodiment 4

In the embodiment, another design method of designing the 2-mode fiber or the 4-mode fiber described in Embodiments 1 to 3 on the basis of specifications as an optical propagation medium such as a laser processing system will be described. The optical fiber design method according to the embodiment includes:

a specification value determining step of determining fiber loss and Raman gain coefficient of a photonic crystal fiber (PCF) to be used, a wavelength of propagating light, a beam quality M² after PCF propagation, a laser output power value, a propagation distance, and a minimum bending radius;

propagation modes maximum number of propagation modes calculating step of calculating the number n of propagation modes that can be propagated by using Mathematical Formula 1;

an effective area calculating step of calculating an effective area A_(eff) from the fiber loss and the Raman gain coefficient by using Mathematical Formula 2;

a fiber structure calculating step of calculating diameter and interval of air holes of the PCF satisfying the A_(eff);

a bending loss calculating step of calculating a bending loss at the minimum bending radius in a PCF having a structure calculated in the fiber structure calculating step and calculating a bending loss at a propagation length from the propagation distance;

a checking step of checking that the bending loss at the propagation length is, for example, less than 0.1 dB and determining the structure of the PCF calculated in the fiber structure calculating step; and

a mode increasing step of, in a case where the bending loss at the propagation length is, for example, 0.1 dB or more in the checking step, repeating the fiber structure calculating step, the bending loss calculating step, and the checking step by increasing the number of modes by one until the number of modes reaches number n of propagation modes.

FIG. 18 is a flowchart illustrating a procedure of the optical fiber design method according to the embodiment.

First, in step S01 (specification value determining step), the used wavelength and target beam quality (M² value) are determined. Next, in step S02 (number-of-propagation modes calculating step), the number of propagation modes that can satisfy the M² value of step S01 is calculated from Mathematical Formula (1) (Mathematical Formula (17) of Non Patent Literature 6), and the number n of propagation modes is determined.

After that, an output power and a propagation distance to be used in step S03 (specification value determining step) are determined, and in step S04 (effective area calculating step), a required effective area (A_(eff)) is calculated by using Mathematical Formula (2) (SRS threshold definition formula (8.1.13) disclosed in Non Patent Literature 7).

Next, in step S05, the number (1+k) of propagatable modes is determined. At this time, in order to improve the beam quality as much as possible, the design is started from the single mode of which the number (1+k) of propagation modes is 1 (k=0). Since step S06 can always be satisfied when k=0, the process proceeds to step S07 (fiber structure calculating step), and a fiber structure satisfying the A_(eff) calculated in step S04 is calculated. In step S08 (bending loss calculating step), the bending loss in the fundamental mode at the minimum bending radius to be used in the fiber structure designed in step S07 is calculated, and in a case where the bending loss exceeds a specified value in step S09, the process returns to step S05, and the number of propagatable modes is incremented by 1, and the procedure up to step S09 is repeated (mode increasing step). At this time, in a case where the number (1+k) of modes exceeds the number n of propagation modes in step S06, there is no solution of the fiber structure satisfying the output power and propagation distance using the set beam quality, so that the process returns to step S01 to review the specification values such as the beam quality (M² value) and repeats the procedure from step S02 to determine the fiber structure.

$\begin{matrix} \left\lbrack {{Numerical}\mspace{14mu}{Expression}\mspace{14mu} 1} \right\rbrack & \; \\ {M^{2} = {C\; n\;\sqrt{\frac{{\left( V_{cutoff}^{{({n + 1})},m} \right)^{2} + {2\left( {n^{2} + 1} \right)}}\;}{3}\;}}} & (1) \end{matrix}$ Herein, V_(cutoff) ^((n+1),m): Cutoff Value of V Number in Immediately Upper Mode LP_(n+1,m) Cn:

${Cn} = {1 + {\frac{1}{2}{\cos(\xi)}}}$ when LP_(1,m), Cn=1 when LP_(n,m) (n≠1), n: Allowable number of propagation modes

$\begin{matrix} \left\lbrack {{Numerical}\mspace{14mu}{Expression}\mspace{14mu} 2} \right\rbrack & \; \\ {P_{tn} = \frac{16A_{eff}}{g_{R}L_{eff}}} & (2) \end{matrix}$ Herein, P_(th): SRS Threshold Value L_(eff): Effective Interaction Length

$L_{eff} = \frac{1 - {\exp\left( {{- \alpha_{p}}L} \right)}}{\alpha_{p}}$ a: Transmission Loss and

$\alpha_{p} = \frac{\alpha}{4.343}$ g_(R): Raman Gain Coefficient

Hereinafter, design examples of the PCF using the above-described design flow are described.

First, in step S01, specification values are determined. Herein, the specification values are as follows.

Fiber loss: 1 dB/km (transmission loss of fiber at the following wavelength)

Raman gain coefficient g_(R): 8.79×10⁻¹² (cm/W)

Used wavelength of propagating light λ: 1070 nm

Beam quality M²: 1.5 or less

Laser output power value: 100 W

Propagation distance: 300 m

Minimum bending radius: 140 m

The Raman gain coefficient gR is calculated by using Mathematical Formula (4) (Mathematical Formula (36) disclosed in Non Patent Literature 8).

$\begin{matrix} \left\lbrack {{Numerical}\mspace{14mu}{Expression}\mspace{14mu} 4} \right\rbrack & \; \\ {g_{R} = {\frac{0.94 \times 10^{- 11}}{\lambda}\left( {1 + {80\Delta}} \right)}} & (4) \end{matrix}$

Herein, Δ is a relative refractive index difference between a core and a clad, and

$\Delta = {\frac{n_{core}^{2} - n_{clad}^{2}}{2n_{core}^{2}}.}$

According to Mathematical Formula (1) in step S02, the allowable number n of propagation modes becomes 2 (cutoff V of LP11=2.405, and cutoff V of LP 21=3.832). However, in a case where the number of propagation modes is 2, it is necessary to set the excitation ratio of the first higher-order mode to 50% or less. Subsequently, in step S04, the necessary A_(e)f_(f) is calculated to be about 160 μm² from Mathematical Formula (2) (the SRS threshold used in Mathematical Formula (2) is the output power determined in step S03). Since the SRS threshold calculated from the Mathematical Formula (2) varies depending on the fiber loss and gR, the necessary A_(eff) also varies. Therefore, the fiber loss and gR are not limited to the contents of the present invention, and are appropriately changed according to the material or the like of the fiber to be used.

Next, the process proceeds to step S05, and first, fiber structure design is performed in a single-mode (the number of propagatable modes is 1). The structure design of PCF can be performed by structure analysis according to a finite element method disclosed in Non Patent Literature 9, an approximate analysis disclosed in Non Patent Literature 10, or the like, and in the embodiment, the structure analysis according to the finite element method is used. The analysis method is not limited to the embodiment, and any method capable of analyzing a structure of a fiber may be appropriately used.

In the embodiment, analysis is performed by using a finite element method. In the 1-cell structure PCF, if it is set that d/Λ=0.42 and Λ=12 μm, A_(eff)=184 μm² is obtained, and thus, the value satisfies the A_(eff) value calculated in step S04. Subsequently, the process proceeds to step S08. In the above-described structure where the minimum bending radius is 140 mm, the bending loss in the fundamental mode is calculated as 1×10⁻⁴ dB/m at R 140 mm. Since the propagation distance is 300 m, the total bending loss is 0.03 dB. In step S09, it is checked whether the bending loss value at the propagation length is 0.1 dB or less. Since the bending loss after the propagation of 300 m is 0.03 dB as described above, the requirement of step S09 is satisfied, and the fiber structure is determined by this structure (step S10).

In addition, since the confinement loss in the first higher-order mode is 6 dB/m or more, this structure operates in a single-mode and causes some axis shift, and even in a case where the first higher-order mode is excited, the first higher-order mode after the propagation of 300 m has a sufficiently small excitation ratio due to the bending loss.

In addition, in a case where k=0 (fundamental mode) and the requirement of step S09 is not satisfied, the process returns to step S05, k is increased (the number of modes is increased), and step S06 to step S09 are repeated. As the number of modes increases, the fiber structure calculated in step S07 is changed, and the bending loss in the fundamental mode is also changed. Steps S05 to S09 are repeated to find the structure with the bending loss satisfying the requirement.

The above description is an example of the structure calculated by using the design flow of FIG. 18, and the fiber parameters may be determined as appropriate by this design flow on the basis of the target beam quality, output power, and propagation distance.

Herein, as expressed in Mathematical Formula (2), the maximum output power (SRS threshold) and the Leff interaction length are inversely proportional to each other. In the optical fiber according to the present invention, since a relatively short propagation distance of 1 km or less is assumed, the Leff and the propagation distance L become equivalent values. Therefore, in this specification, output power performance is described as a product (kW·m) of an output power and a propagation distance. In addition, the propagation distance is not limited to 1 km or less, and the propagation distance can be similarly applied as long as the Leff and the L can be regarded as equivalent to each other.

Embodiments 5 to 8

Embodiments 5 to 8 will be described with reference to FIGS. 19 to 22. The figures illustrate regions where, as desired PCF parameters, d/Λ is represented by the horizontal axis and Λ is represented by the vertical axis. Herein, (a) in the condition of M²≤2.0, the coordinates are set to

A1(0.42, 16.88),

A2(0.48, 25.31),

A3(0.57, 40.00),

B(0.42, 10.94),

C1(0.60, 15.63),

C2(0.69, 31.88),

C3(0.76, 47.81),

D(0.74, 43.12),

E(0.75, 44.38),

F(0.81, 60.63),

G(0.85, 60.63),

H(0.85, 77.50), and

I(0.90, 91.88),

(b) in the condition of M²≤3.3, the coordinates are set to

A1(0.42, 16.88),

A2(0.47, 25.31),

A3(0.56, 40.00),

B(0.42, 10.94),

C1(0.75, 15.00),

C2(0.78, 35.00),

C3(0.90, 54.38),

D1(0.75, 20.00),

D2(0.80, 35.93),

E(0.80, 45.63),

F(0.83, 51.56), and

I(0.90, 91.88),

and, (c) in the condition where the number of propagation modes is 4 or less, the coordinates are set to

A1(0.42, 16.88),

A2(0.48, 25.31),

A3(0.57, 40.00),

A4(0.75, 68.36),

B(0.42, 10.94),

C(0.75, 14.24),

D(0.75, 12.10),

E(0.79, 20.00),

F(0.85, 30.00),

F1(0.85, 36.37),

G(0.85, 41.58),

H(0.89, 50.00),

H1(0.89, 54.37),

I(0.89, 58.95),

J(0.90, 60.0),

J1(0.90, 77.07), and

K(0.90, 91.88).

Embodiment 5

[1-Cell Structure, 30 kW·m Propagation]

In the embodiment, ranges of PCF parameters (Λ and d) of a 1-cell structure in FIG. 4 enabling 30 kW·m propagation will be described. FIG. 19 is a diagram for explaining a relationship between d/Λ, Λ, and A_(eff) (160 μm² or more) that satisfy (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is 4 or less when light having a wavelength of 1070 nm is propagated. The plotted region (hatched region) has a structure capable of performing 30 kW·m propagation in a PCF having a 1-cell structure.

More specifically, (a) in a case where M²≤2.0, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A1, B, C1, C2, D, E, C3, F, G, H, and I. In addition, (b) in a case where M²≤3.3, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A1, B, C1, D1, C2, D2, E, F, C3, and I. On the other hand, (c) in a case where the number of propagation modes is 4 or less, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A1, B, C, D, E, F, G, H, I, J, and K.

Embodiment 6

[1-Cell Structure, 150 kW·m Propagation]

In the embodiment, ranges of PCF parameters (A and d) of a 1-cell structure in FIG. 4 enabling 150 kW·m propagation will be described. FIG. 20 is a diagram for explaining a relationship between d/Λ, Λ, and A_(eff) (800 μm² or more) that satisfy (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is 4 or less when light having a wavelength of 1070 nm is propagated. The plotted region (hatched region) has a structure capable of performing 150 kW·m propagation in a PCF having a 1-cell structure.

More specifically, (a) in a case where M²≤2.0, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A2, C2, D, E, C3, F, G, H, and I. In addition, (b) in a case where M²≤3.3, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A2, C2, D2, E, F, C3, and I. On the other hand, (c) in a case where the number of propagation modes is 4 or less, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A2, F1, G, H, I, J, and K.

Embodiment 7

[1-Cell Structure, 300 kW·m Propagation]

In the embodiment, ranges of PCF parameters (A and d) of a 1-cell structure in FIG. 4 that enables 300 kW·m propagation will be described. FIG. 21 is a diagram for explaining a relationship between d/Λ, Λ, and A_(eff) (1600 μm² or more) that satisfy (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is 4 or less when light having a wavelength of 1070 nm is propagated. The plotted region (hatched region) has a structure capable of performing 300 kW·m propagation in a PCF having a 1-cell structure.

More specifically, (a) in a case where M²≤2.0, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A3, C3, F, G, H, and I. In addition, (b) in a case where M²≤3.3, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A3, C3, and I. On the other hand, (c) in a case where the number of propagation modes is 4 or less, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A3, H1, I, J, and K.

Embodiment 8

[1-Cell Structure, 600 kW·m Propagation]

In the embodiment, ranges of PCF parameters (A and d) of a 1-cell structure in FIG. 4 that enables 600 kW·m propagation will be described. FIG. 22 is a diagram for explaining a relationship between d/Λ, Λ, and A_(eff) (3200 m² or more) that satisfy (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is 4 or less when light having a wavelength of 1070 nm is propagated. The plotted region (hatched region) has a structure capable of performing 600 kW·m propagation in a PCF having a 1-cell structure.

More specifically, in the case of the condition that the number of propagation modes is 4 or less, d/Λ and Λ disposed inside an area surrounded by a polygon having A4, J1, and K as its vertexes are set.

Embodiments 9 to 13

Embodiments 9 to 13 will be described with reference to FIGS. 23 to 27. The figures illustrate regions when, as desired PCF parameters, d/Λ is represented by the horizontal axis and Λ is represented by the vertical axis. Herein, (a) in M²≤2.0, the coordinates are set to

A1(0.20, 10.98),

A2(0.21, 11.77),

A3(0.27, 16.06),

A4(0.40, 24.46),

A5(0.53, 32.87),

B(0.20, 4.95),

C1(0.25, 5.27),

C2(0.40, 12.88),

C3(0.60, 20.34),

C4(0.68, 29.56),

C5(0.72, 36.35),

D(0.29, 9.87),

E(0.40, 12.25),

F(0.40, 13.52),

G(0.49, 14.15),

H(0.49, 15.74),

I(0.50, 18.12),

J(0.58, 18.12),

K(0.58, 19.86),

L(0.60, 23.03),

M(0.68, 23.99),

N(0.68, 31.60),

O(0.79, 48.73), and

P(0.80, 50.00),

(b) in M²≤3.3, the coordinates are set to

A1(0.20, 10.98),

A2(0.21, 11.77),

A3(0.27, 16.06),

A4(0.40, 24.78),

A5(0.53, 32.87),

B(0.20, 5.11),

C1(0.40, 5.90),

C2(0.50, 13.68),

C3(0.60, 20.18),

C4(0.70, 30.01),

C5(0.73, 37.00),

D(0.40, 10.03),

E(0.50, 11.93),

F(0.50, 14.47),

C3(0.60, 20.18),

G(0.69, 22.08),

H(0.68, 23.67,),

I(0.70, 24.30),

J(0.70, 32.87), and

K(0.80, 50.00),

and (c) in the condition where the number of propagation modes is 4 or less, the coordinates are set to

A1 (0.20, 10.98),

A2(0.21, 11.77),

A3(0.27, 16.06),

A4(0.40, 24.78),

A5(0.53, 32.87),

B(0.20, 5.11),

C(0.50, 6.23),

D(0.50, 10.00),

E(0.60, 15.18),

F(0.60, 17.76),

G(0.65, 20.12),

H(0.70, 20.35),

I(0.79, 25.06),

J(0.79, 29.53),

K(0.78, 29.76),

K1(0.78, 30.66),

L(0.78, 38.29),

M(0.80, 40.12), and

N(0.80, 50.00).

Embodiment 9

[7-Cell Structure, 30 kW*m Propagation]

In the embodiment, ranges of PCF parameters (Λ and d) of a 7-cell structure in FIG. 10 that enables 30 kW·m propagation will be described. FIG. 23 is a diagram for explaining a relationship between d/Λ, Λ, and A_(eff) (160 μm² or more) that satisfy (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is 4 or less when light having a wavelength of 1070 nm is propagated. The plotted region (hatched region) has a structure capable of performing 30 kW·m propagation in a PCF having a 7-cell structure.

More specifically, (a) in a case where M²≤2.0, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A1, B, C1, D, E, F, G, H, I, J, K, C3, L, M, N, O, and P. In addition, (b) in a case where M²≤3.3, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A1, B, C1, D, E, F, C3, G, H, I, J, and K. On the other hand, (c) in a case where the number of propagation modes is 4 or less, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A1, B, C, D, E, F, G, H, I, J, K, L, M, and N.

Embodiment 10

[7-Cell Structure, 150 kW·m Propagation]

In the embodiment, ranges of PCF parameters (Λ and d) of a 7-cell structure of FIG. 11 that enables 150 kW·m propagation will be described. FIG. 24 is a diagram for explaining a relationship between d/Λ, Λ, and A_(eff) (800 μm² or more) that satisfy (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is 4 or less when light having a wavelength of 1070 nm is propagated. The plotted area (hatched area) has a structure capable of performing 150 kW·m propagation in a PCF having a 7-cell structure.

More specifically, (a) in a case where M²≤2.0, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A2, C2, F, G, H, I, J, K, C3, L, M, N, O, and P. In addition, (b) in a case where M²≤3.3, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A2, C2, F, C3, G, H, I, J, and K. On the other hand, (c) in a case where the number of propagation modes is 4 or less, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A2, E, F, G, H, I, J, K, L, M, and N.

Embodiment 11

[7-Cell Structure, 300 kW·m Propagation]

In the embodiment, ranges of PCF parameters (Λ and d) of a 7-cell structure in FIG. 11 that enables 300 kW·m propagation will be described. FIG. 25 is a diagram for explaining a relationship between d/Λ, Λ, and A_(eff) (1600 μm² or more) that satisfy (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is 4 or less when light having a wavelength of 1070 nm is propagated. The plotted region (hatched region) has a structure capable of performing 300 kW·m propagation in a PCF having a 7-cell structure.

More specifically, (a) in a case where M²≤2.0, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A3, C3, L, M, N, O, and P. In addition, (b) in a case where M²≤3.3, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A3, C3, G, H, I, J, and K. On the other hand, (c) in a case where the number of propagation modes is 4 or less, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A3, G, H, I, J, K, L, M, and N.

Embodiment 12

[7-Cell Structure, 600 kW·m Propagation]

In the embodiment, ranges of PCF parameters (Λ and d) of a 7-cell structure of FIG. 11 that enables 600 kW·m propagation will be described. FIG. 26 is a diagram for explaining a relationship between d/Λ, Λ, and A_(eff) (3200 μm² or more) that satisfy (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is 4 or less when light having a wavelength of 1070 nm is propagated. The plotted region (hatched region) has a structure capable of performing 600 kW·m propagation in a PCF having a 7-cell structure.

More specifically, (a) in a case where M²≤2.0, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A4, C4, N, O, and P. In addition, (b) in a case where M²≤3.3, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A4, C4, J, and K. On the other hand, (c) in a case where the number of propagation modes is 4 or less, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A4, K1, L, M, and N.

Embodiment 13

[7-Cell Structure, 900 kW·m Propagation]

In the embodiment, ranges of PCF parameters (A and d) of a 7-cell structure of FIG. 11 that enables 900 kW·m propagation will be described. FIG. 27 is a diagram for explaining a relationship between d/Λ, Λ, and A_(eff) (4800 μm² or more) that satisfy (a) M²≤2.0, (b) M²≤3.3, and (c) the number of propagation modes is 4 or less when light having a wavelength of 1070 nm is propagated. The plotted area (hatched area) has a structure capable of performing 900 kW·m propagation in a PCF having a 7-cell structure.

More specifically, if (a) M²≤2.0, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A5, C5, O, and P. In addition, (b) in a case where M²≤3.3, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A5, C5, and K. On the other hand, (c) in a case where the number of propagation modes is 4 or less, the air holes are set to d/Λ and Λ in a region surrounded by a polygon having vertices A5, K, M, and N.

Embodiment 14

FIG. 28 illustrates an example of the structure of a PCF according to this embodiment. The PCF according to this embodiment represents an optical fiber structure configured by air holes corresponding to a number smaller than the number of the air holes of the PCF illustrated in FIG. 4. According to the structure of the PCF illustrated in FIG. 28, in a core area, similarly to the case illustrated in FIG. 4, there is one defect in the air holes, and the number of the air holes is 12. A PCF has optical characteristics influenced by accuracy of the positions and the sizes of individual air holes, and the difficulty in the manufacturing or the degradation of the yield of the structure becomes remarkable as the number of the air holes is increased. According to the structure illustrated in FIG. 28, the guidance of an optical wave is realized using a simple structure in which the number of air holes is 12 or less, and accordingly, the mass productivity is high, and the controllability of optical characteristics at the time of manufacturing is improved, which is preferable. In addition, also in the structure of the PCF illustrated in FIG. 28, high-power light of high quality having M2 to be 2.0 or less in a predetermined design range and the kW class can be transmitted.

FIGS. 29 to 32 illustrate examples of design of a PCF in which the number of air holes is 12. FIG. 29 illustrates a structure range in which the bending loss for the basic mode is 0.1 dB/m or less. FIGS. 30, 31, and 32 respectively illustrate bending loss values for an LP11 mode, an LP21 mode, and an LP02 mode, and the loss of a target mode is sufficiently large in an area (a bending loss of 1 dB/m or more) surrounded inside the range, so as to be leaked.

Here, by blocking the LP02 mode and causing an axial deviation amount between a laser oscillating unit and the PCF to be a predetermined value or less, output light of high quality and high power is acquired. Accordingly, in the range, in which FIGS. 29 and 32 are overlapped, illustrated in FIG. 33, the object can be achieved. In other words, a structure in which A is 50 μm or more and 65 μm or less, and d/Λ is 0.79 or more and 0.88 or less or a structure in which Λ is 50 μm or less, and d/Λ is 0.7 or more and 0.79 or less is appropriate. At this time, since the number of air holes is 12 which is small, the yield of the manufacturing and the manufacturing accuracy are very good, and output light of M2 of 2.0 or less and the kW class is acquired, which is preferable.

In addition, in this embodiment, while the predetermined number of modes is three, LP01, LP11, and LP21 that are basic modes are configured to be propagated, but LP02 is not configured to be propagated, the present invention is not limited thereto. For example, it may be configured such that the predetermined number of modes is two, the LP01 mode and the LP11 mode are propagated, but the LP21 mode and higher modes are not propagated. In such a case, effects similar to those of this embodiment can be acquired.

For example, in FIGS. 30 and 32, a structure in which the LP11 mode and the LP21 mode are leaked is illustrated, and, the number of modes can be configured to be two by using an area in which FIGS. 29 and 30 are overlapped, and the number of modes can be configured to be three by using an area in which FIGS. 29 and 32 are overlapped. In other words, by configuring a structure in which Λ is 30 μm or more and 55 μm or less and d/Λ is 0.70 or more and 0.79 or less, Λ is 55 μm or more and 65 μm or less and d/Λ is 0.79 or more and 0.83 or less, Λ is 57 μm or more and 65 μm or less and d/Λ is 0.83 or more and 0.88 or less, or Λ is 59 μm or more and 68 μm or less and d/Λ is 0.88 or more and 0.89 or less, the number of modes can be configured to be two. In addition, by configuring a structure in which Λ is 52 μm or more and 65 μm or less and d/Λ is 0.79 or more and 0.88 or less, or Λ is 52 μm or less and d/Λ is 0.7 or more and 0.77 or less, the number of modes can be configured to be three.

Effects of Invention

In the present invention, with respect to required output power and propagation distance, a definition formula of an SRS threshold is used, and with respect to the required beam quality, an M2 value in a case where a propagation mode is uniformly excited from a bending loss and the number of propagatable modes is used as a threshold, so that it is possible to design a fiber structure satisfying the above conditions. Furthermore, it is possible to clarify a specific structure of high-quality high-power propagation optical fiber by using the design flow. As a specific design example, a structural example of a PCF is illustrated.

As described above, according to an optical fiber and an optical fiber design method according to the present invention, it is possible to provide an optical fiber capable of ensuring an output power with respect to a propagation length at a desired beam quality which cannot be realized in the design of the related art.

(Function)

The fiber structure designed by using the design flow used in the present invention can satisfy required output power, propagation distance and beam quality. Even in a region of an output power of a fiber laser which has been realized only with a multi-mode with an M² value of 8 or more, it is possible to realize use of light having high quality beam quality for a desired propagation distance by using an optical fiber with an M² value of less than 8 for a fiber laser.

INDUSTRIAL APPLICABILITY

The present invention can be applied to the field of laser processing using fiber laser.

REFERENCE SIGNS LIST

-   1: PCF -   2: Empty hole 

The invention claimed is:
 1. A method of designing an optical fiber, the method comprising: determining a fiber loss and a Raman gain coefficient of a photonic crystal fiber (PCF) to be used, a wavelength of propagating light, a beam quality M² after PCF propagation, a laser output power value, a propagation distance, and a minimum bending radius; calculating a largest number n of propagation modes that can be propagated by using Mathematical Formula 1; calculating an effective area A_(eff) from the fiber loss and the Raman gain coefficient by using Mathematical Formula 2; calculating a diameter and an interval of air holes of the PCF satisfying the A_(eff); calculating a bending loss at the minimum bending radius in the PCF having a structure calculated in the calculating of a diameter and an interval and calculating a bending loss of a propagation length from the propagation distance; checking that the bending loss at the propagation length is a predetermined value or less and determining the structure of the PCF calculated in the calculating of a diameter and an interval; increasing the number of modes by one in a case where the bending loss of the propagation length is a predetermined value or more in the checking and repeating the calculating of a diameter and an interval, the calculating of a bending loss, and the checking until the number of modes reaches the largest number n of propagation modes; wherein Mathematical Formula 1 and Mathematical Formula 2 are expressed as: $\begin{matrix} \left\lbrack {{Numerical}\mspace{14mu}{Expression}\mspace{14mu} 1} \right\rbrack & \; \\ {M^{2} = {{Cn}\sqrt{\frac{\left( V_{cutoff}^{{({n + 1})},m} \right)^{2} + {2\left( {n^{2} - 1} \right)}}{3}}}} & (1) \end{matrix}$ Herein, V_(cutoff) ^((n+1),m): Cutoff Value of V Number in Immediately Upper Mode LP_(n+1,m) Cn: ${{Cn} = {1 + {\frac{1}{2}{\cos(\xi)}}}}\;$  when LP_(1,m), Cn=1 when LP_(n,m)(n≠1), n: Allowable number of propagation modes $\begin{matrix} \left\lbrack {{Numerical}\mspace{14mu}{Expression}\mspace{14mu} 2} \right\rbrack & \; \\ {P_{tn} = \frac{16A_{eff}}{g_{R}L_{eff}}} & (2) \end{matrix}$ Herein, P_(th): SRS Threshold Value L_(eff): Effective Interaction Length ${L_{eff} = \frac{1 - {\exp\left( {{- \alpha_{p}}L} \right)}}{\alpha_{p}}}\;$ α: Transmission Loss and $\alpha_{p} = \frac{\alpha}{4.343}$ g_(R): Raman Gain Coefficient.
 2. A method of designing an optical fiber, the method comprising: determining a fiber loss and a Raman gain coefficient of a photonic crystal fiber (PCF) to be used, a wavelength of propagating light, a beam quality M² after PCF propagation, a laser output power value, a propagation distance, and a minimum bending radius; calculating a largest number n of propagation modes that can be propagated by using Mathematical Formula 1; calculating an effective area A_(eff) from the fiber loss and the Raman gain coefficient by using Mathematical Formula 2; calculating a diameter d and an interval Λ of air holes of the PCF having the A_(eff) or more and plotting dots having the A_(eff) or more on a graph having a horizontal axis set as d/Λ and a vertical axis set as Λ; calculating a bending loss at a minimum bending radius of a minimum high-order mode cut off by the PCF based on the diameter d and the interval Λ of the air holes of the PCF and plotting points having the bending loss of 1 dB/m or more on a graph having a horizontal axis set as d/Λ and a vertical axis set as Λ; detecting an overlapping range in which an area of the points plotted on the graph in the calculating a diameter and an interval and an area of the point plotted on the graph in the calculating of a bending loss are overlapped and determining a PCF structure having air holes of the diameter d and the interval Λ that are in the overlapping range; wherein Mathematical Formula 1 and Mathematical Formula 2 are expressed as: $\begin{matrix} \left\lbrack {{Numerical}\mspace{14mu}{Expression}\mspace{14mu} 1} \right\rbrack & \; \\ {M^{2} = {{Cn}\sqrt{\frac{\left( V_{cutoff}^{{({n + 1})},m} \right)^{2} + {2\left( {n^{2} - 1} \right)}}{3}}}} & (1) \end{matrix}$ Herein, V_(cutoff) ^((n+1),m): Cutoff Value of V Number in Immediately Upper Mode LP_(n+1,m) Cn: ${{Cn} = {1 + {\frac{1}{2}{\cos(\xi)}}}}\;$  when LP_(1,m), Cn=1 when LP_(n,m) (n≠1), n: Allowable number of propagation modes $\begin{matrix} \left\lbrack {{Numerical}\mspace{14mu}{Expression}\mspace{14mu} 2} \right\rbrack & \; \\ {P_{th} = \frac{16A_{eff}}{g_{R}L_{eff}}} & (2) \end{matrix}$ Herein, P_(th): SRS Threshold Value L_(eff): Effective Interaction Length ${L_{eff} = \frac{1 - {\exp\left( {{- \alpha_{p}}L} \right)}}{\alpha_{p}}}\;$ α: Transmission Loss and $\alpha_{p} = \frac{\alpha}{4.343}$ g_(R): Raman Gain Coefficient. 